Various techniques have been suggested as to the lithographic printing plate precursor capable of image-forming by heat and directly loadable on a printer without undergoing development processing. One promising method is a method of utilizing abrasion, wherein a lithographic printing plate precursor is subjected to exposure with solid state high output infrared-ray lasers such as a semiconductor laser or a YAG laser to make the irradiated part generate heat with a light-to-heat conversion material, thereby cracking evaporation is caused.
That is, this is a method of providing a hydrophilic layer on a lipophilic substrate or a substrate having a lipophilic layer and removing the hydrophilic layer by abrasion.
WO 94/18005 discloses a printing plate comprising a laser beam-absorbing lipophilic layer having provided thereon a crosslinked hydrophilic layer, wherein the hydrophilic layer is removed by abrasion. This hydrophilic layer comprises polyvinyl alcohol crosslinked with the hydrolyzate of tetraethoxysilane and titanium dioxide particles contained therein, which intends to improve the strength of the hydrophilic layer. The impression capability of a lithographic printing plate precursor is improved by this technique, however, since the polyvinyl alcohol having hydrocarbon groups and not always having high hydrophilic property accounts for 48 wt % of the hydrophilic layer, the resistance to staining is not still sufficient and further improvement is required.
There are disclosed in WO 98/40212, WO 99/19143 and WO 99/19144 lithographic printing plate precursors directly loadable on a printer without subjecting to development, which comprises a substrate having coated thereon an ink-receptive layer, and a hydrophilic layer comprising, as a main component, a colloid such as silica, crosslinked with a crosslinking agent such as aminopropyltriethoxysilane. This hydrophilic layer contains hydrocarbon groups as small as possible to heighten the resistance to printing staining and is improved in the impression capability by crosslinking the colloid with a crosslinking agent, but the impression capability is several thousands, which is still insufficient.
The digital direct processing-free printing plate which utilizes abrasion has big advantages of rationalization of printing and the reduction of wastes such that plate-making can be performed directly from a camera-ready copy without the use of a film, the printing plate can be loaded on a printer as it is and printing can be performed immediately. However, due to the difficulty of processing-free technique, either the resistance to staining or the impression capability, both of which are fundamentals of the printing, is liable to be damaged, therefore, a technique which makes both compatible has not yet been developed.